1. Field of the Invention
This invention relates to a magnetic recording medium in which a magnetic wall is caused to displace to thereby reproduce information and a recording-reproducing method therefor.
2. Related Background Art
Attention is paid to magnetic recording media such as a magnetic recording medium and a magneto-optical recording medium for recording information thereon based on an orientation state of magnetization of a magnetic material and a recording-reproducing apparatus as a high density recording system capable of rewriting. In recent years, a demand has arisen for increasing the recording density of these magnetic recording media to thereby provide recording media of further large capacity.
In the magneto-optical recording system, the heat energy of a semiconductor laser is utilized to write a magnetic domain into magnetic thin film and record information, and the recorded information is read out by the use of the magneto-optical effect. Generally, the linear recording density of an optical recording medium depends greatly on the laser wavelength of a reproducing optical system and the numerical aperture NA of an objective lens. That is, when the laser wavelength xcex of the reproducing optical system and the numerical aperture NA of the objective lens are determined, the diameter of a beam waist is determined and therefore, the spatial frequency of a recording pit capable of reproducing a signal is limited to the order of 2NA/xcex.
Accordingly, to realize higher density in a conventional optical disc, it is necessary to shorten the laser wavelength of the reproducing optical system or make the numerical aperture of the objective lens great. However, it is not easy due to the problems of the efficiency, heat generation, etc. of the element to shorten the laser wavelength, and if the numerical aperture of the objective lens is made great, the problem will arise that the depth of focus becomes shallow and the requirement for mechanical accuracy becomes severe.
Therefore, various so-called super-resolving techniques for contriving the construction of the recording medium and the reproducing method and improving the recording density without changing the laser wavelength and the numerical aperture of the objective lens have been developed.
For example, Japanese Laid-Open Patent Application No. 3-93058, proposes a signal reproducing method of effecting signal recording on a recording holding layer of multilayer film comprising a reproducing layer and a recording holding layer magnetically coupled together, uniformizing the direction of magnetization of the reproducing layer, thereafter irradiating the reproducing layer with a laser beam and heating it, and reading a signal recorded on the recording holding layer while transferring the signal to the temperature-increased area of the reproducing layer.
According to this method, relative to the spot diameter of a reproducing laser, an area which is heated by this laser and reaches a transfer temperature and in which a signal is detected can be limited to a smaller area and therefore, the intersymbol interference during reproduction can be decreased and a signal of a period less than the diffraction limit of the light becomes reproducible.
However, the conventional super-resolving system has adopted a method of masking part of reproducing light, and limiting an aperture for substantially reading a pit to a small area to thereby increase the resolving capability. This has led to the problem that the light of the masked part becomes useless and the reproduction signal amplitude becomes small. That is, the light of the masked part does not contribute to the reproduction signal. Therefore, the more the aperture is narrowed in an attempt to increase the resolving power, the more the light effectively used decreases and the lower becomes the signal level.
In view of such a problem, Japanese Laid-Open Patent Application No. 6-290496 already proposes a method of displacing a magnetic wall present in the boundary portion of a recording mark by a temperature gradient by the use of a special magnetic recording medium, and detecting this displacement of the magnetic wall to thereby reproduce a high density recording signal.
This method, however, is a novel reproducing method entirely differing from the conventional reproducing system and therefore, the detailed conditions thereof have included many unknown portions.
The present inventor has eagerly repeated investigations about the above-described magnetic recording medium and a reproducing method therefor and, as a result, the inventor has obtained more detailed findings about the property of the material of the recording medium and a reproducing condition therefor. The present invention has as its object to disclose conditions that are more proper for stably realizing the function shown in the above-mentioned Japanese Laid-Open Patent Application No. 6-290496, to thereby provide a magnetic recording medium capable of effecting high density recording and reproduction and a reproducing method therefor.
The above object is achieved by satisfying, in a magnetic recording medium having at least first, second and third magnetic layers laminated in succession,
2Ms1*Hw1 less than "sgr"w13/h1
and
2Ms3*Hw3 greater than "sgr"w13/h3
at least at room temperature when the magnetic wall energy density, saturation magnetization, magnetic wall coercivity and film thickness of the first magnetic layer at a temperature T represented by cgs unit system are defined as "sgr"1, Ms1, Hw1 and h1, respectively, the magnetic wall energy density, saturation magnetization, magnetic wall coercivity and film thickness of the third magnetic layer are defined as "sgr"3, Ms3, Hw3 and h3, respectively, the interface magnetic wall energy density between the first magnetic layer and the third magnetic layer is defined as "sgr"w13, and minimum temperature Ts is defined as a temperature at which "sgr"w13 is 0 erg/cm2,
and satisfying
k1(T) less than k3(T)
and
            ∫      Ts      Tp        ⁢                  1                  k1          ⁢                      (            T            )                              ⁢              xe2x80x83            ⁢              ⅆ        T               greater than       0.2    xc3x97          10              -        4            
within a temperature range of at least Ts to Tp when a suitable temperature Tp is chosen to a temperature range greater than the temperature Ts and lower by 10xc2x0 C. or more than the Curie temperature Tc1 of the first magnetic layer.
In the foregoing, it is to be understood that
k1(T)=(2Ms1*Hw1+"sgr"w13/h1)/|d"sgr"1(T)|
k3(T)=(2Ms1*Hw3xe2x88x92"sgr"w13/h3)/|d"sgr"3/dT|.
Also, the above object is achieved by satisfying, in a magnetic recording medium having at least first, second and third magnetic layers laminated in succession, said first magnetic layer being comprised of n constituent layers comprising a layer 11, a layer 12, a layer 1n in succession from the side near said second magnetic layer,
xe2x80x83Tc11 less than Tc12 less than  . . .  less than Tc1n
when the interface magnetic wall energy density between said first magnetic layer and said third magnetic layer is defined as "sgr"w13 and the minimum temperature Ts is defined as a temperature at which w13 is 0 erg/cm2, and satisfying
2Ms1*Hw1 less than "sgr"w13/h1
and
2Ms3*Hw3 greater than "sgr""sgr"w13/h3
at least at room temperature, and satisfying
k1(T) less than k3(T)
            ∫      Ts      Tp        ⁢                  1                  k1          ⁢                      (            T            )                              ⁢              xe2x80x83            ⁢              ⅆ        T               greater than       0.2    xc3x97          10              -        4            
within a temperature range of at least Ts to Tp when a suitable temperature Tp is chosen to a temperature range greater than the temperature Ts and lower by 10xc2x0 C. or more than the Curie temperature of said 1n-th constituent layer.
Assuming in the foregoing that in the 1n-the constitutent layer the Curie temperature is Tc1i and the magnetic wall energy density, saturation magnetization, magnetic wall coercivity and film thickness at a temperature T represented by cgs unit system are defined as "sgr"1i, Ms1i, Hw1i and h1i, respectively (where i represents one of integers 1 to n),             σ      ⁢              xe2x80x83            ⁢      1        =                  ∑                  i          =          1                n            ⁢                        (                      σ            ⁢                          xe2x80x83                        ⁢            l            ⁢                          xe2x80x83                        ⁢            i            *            h1i                    )                /                              ∑                          i              =              1                        n                    ⁢          h1i                                Ms1      *      Hw1        =                  ∑                  i          =          1                n            ⁢                        (                      Ms1i            *            Hw1i            *            h1i                    )                /                              ∑                          i              =              1                        n                    ⁢          h1i                          h1    =                  ∑                  i          =          1                n            ⁢      h1i      
and assuming that the magnetic wall energy density, saturation magnetization, magnetic wall coercivity and film thickness of the third magnetic layer are defined as "sgr"3, Ms3, Hw3 and h3, respectively,
k1(T)=(2Ms1*Hw1+"sgr"w13/h1)/|d"sgr"1/dT|
k3(T)=(2Ms3*Hw3xe2x88x92"sgr"w13/h3)/|d"sgr"3/dT|.
Also, the above object is achieved by forming, in a recording method of recording information on the above-described magnetic recording medium, a recording mark corresponding to the information on recording tracks with a width over the full width of the recording tracks.
Further, the above object is achieved by a reproducing method of reproducing information recorded on the above-described magnetic recording medium, including the step of applying a temperature profile on the medium to form a temperature gradient G(T) satisfying
G(T) less than k3(T), and satisfying
G(T) greater than k1(T)
and
            ∫      Ts              Tp        xe2x80x2              ⁢                  1                  G          ⁡                      (            T            )                              ⁢              xe2x80x83            ⁢              ⅆ        T               greater than       0.2    xc3x97          10              -        4            
within a temperature range of at least Ts to Tpxe2x80x2 when a suitable temperature Tpxe2x80x2 is chosen to a temperature range greater than the temperature Ts and lower by 10xc2x0 C. or more than the Curie temperature of said first magnetic layer, and the step of scanning said temperature profile on the surface of the medium, displacing only the magnetic wall in the first magnetic layer toward the high temperature side while keeping the magnetic wall in the third magnetic layer at the moment when the magnetic walls enter a temperature area where the temperature is higher than Ts, and detecting the displacement of this magnetic wall.